Topic 6 Flashcards
(19 cards)
Streak plating on selective media (5)
Flame inoculating loop
First streaks of sample in straight lines on surface of sterile agar
Repeat at least three streaks
Second and third streak must pass through lines of previous streak
Incubate and take off individual colonies with loop
dilution plating
+ (3)
- (3)
+ Counts living cells
+ Provides a direct count
+ Involves use of easily available apparatus
- Takes longer to obtain results as incubation period is required
- Need to calculate original concentration of bacterial cells
- Error if colonies overlap
Optical methods
+ (2)
- (3)
+ Quick to obtain results
+ Uses a calibration curve turbidity and number of cells present
- Counts all cells (dead or living)
- Turbidity reading
- Requires expensive colorimeter, error if tube is not shaken
phases of bacterial growth
Lag phase: bacteria are adapting to new environment and so are not reproducing at their maximum rate
Log phase: rate of reproduction is close to or at its theoretical maximum, doubling per time
o Eventually slows down – reduction of available nutrients and build up of waste products which are toxic so inhibits further growth e.g. CO2 causes pH to fall
Stationary phase: total growth rate is zero as the number of new cells formed by binary fission is equal to the number of cells dying
Death phase: reproduction has almost stopped and increasing death rate
primary infection
bacteria invades host cells and multiplies slowly
E.g. Mycobacterium tuberculosis affects respiratory system as it damages lung tissue
endotoxins
lipopolysaccharides produced by gram-negative bacteria and part of their cell wall effects the site around infection
E.g. Salmonella, food poisoning present after 12hours
Inflammatory response in epithelial cells of intestine reduces water absorption
exotoxins
soluble proteins produced by gram-positive and gram-negative bacteria effects sites away from infection
damages cell membranes or act as competitive inhibitors to neurotransmitters or as a poison to cells
E.g. Staphylococcus, food poisoning within a few hours
selective toxicity
interfere with metabolism or function of pathogen with minimal damage to cells of host
bacteriostatic
antibiotics that completely inhibits the growth of bacteria
Inhibit protein synthesis by preventing transcription and translation, interrupt metabolic pathways
E.g. Tetracycline treats acne, urinary/ respiratory tract infections and Chlamydia
bactericidal
antibiotics that destroy bacteria
Prevent formation of crosslinking in cell walls, damage cell membrane, inhibit DNA coiling so it no longer fits
E.g. Penicillin treats skin infections, chest infections, urinary tract infections
how does skin flora kill pathogens
competition for nutrients
release chemicals/ lipids/ enzymes
macrophages as phagocytes
agranular leucocytes derived from monocytes
have a large capacity for ingesting pathogens and ability to renew lysosomes
neutrophils
granular leucocytes
can only digest a few pathogens before dying as they are unable to renew lysosomes so will be used up
phagocytosis
- Phagocyte is attracted to the chemicals produced by pathogen and attaches to its surface
- Phagocyte engulfs pathogen forming a phagocytic vesicle (phagosome) which are formed from the cell membrane of the phagocyte by extensions of the cytoplasm
- Lysosomes (bags of enzymes) move and fuse with phagosome
- Lysozymes digest pathogen by hydrolysing cell wall
- Products are absorbed into the cytoplasm of the phagocyte, waste products leave and pathogen antigens are presented on the surface of the phagocyte
- Phagocytes release cytokines to stimulate other phagocytes and specific immune response
cytokines
stimulates phagocytes to move to site of infection
opsosnins
chemical that binds to pathogens to form antigen-antibody complexes so they are more recognisable to phagocytes
T cell response
cell mediated immunity
- Phagocytes engulf pathogen and presents their antigens on MHC on their surface
- Receptors on specific helper T cells (TH) bind to antigen on antigen-presenting cells as they have complementary shapes
- This attachment releases cytokines which activate the T-helper cells to divide rapidly by mitosis
- Cloned T cells:
- Develop into memory T cells allowing a rapid response to same pathogen
- T helper cells secrete opsonins to stimulate phagocytosis and stimulates plasma cells to divide and produce antibodies (agglutination)
- Activates cytotoxic T cells (Tc or T-killer cells) that kill infected cells by producing a protein called perforin which makes holes in cell membranes so it becomes freely permeable
B cell response
humoral immunity
1. B cell with specific antibody fits onto the antigen on the surface of the foreign cell due to complementary protein shapes
2. Antigen enters B cell by endocytosis and combines with major histocompatibility complex (MHC) proteins to form antigen-MHC protein complex and is presented on the surface of the B cell (antigen-presenting cell – APC)
3. Receptors on T-helper cells bind to these antigens secreting cytokines
4. Specific B cell divides by mitosis so all clones produce antibodies (immunoglobulins) specific to the foreign antigen
o Clonal selection: ability to respond rapidly to specific antigens by cloning complementary antibodies
5. Antibodies attach to antigen on pathogen and destroys it
6. B cells become either B effector cells or memory cells
o B effector cells differentiate into clones of plasma cells secrete antibodies into the blood plasma for the primary immune response
o Memory cells will divide rapidly if the same specific antigen is present again into plasma cells and more memory cells for secondary immune response
evolutionary race between antibiotics and host
mutation causes a change in antigens
no secondary immune response as memory cells do not recognise antigen
another primary response is required
use of antibiotics increase proportion of resistance in population as it increases selection pressure
